The disclosures, including the claims, figures and/or drawings, of each and every patent, patent application, and publication cited herein are hereby incorporated herein by reference in their entireties.
The following description includes information that may be useful in understanding the present invention. It is not an admission that any of the information provided herein is prior art or relevant to the presently claimed inventions, or that any publication specifically or implicitly referenced is prior art.
There are numerous steps in the development of any novel, desirable plant germplasm. Plant breeding begins with the analysis and definition of problems and weaknesses of the current germplasm, the establishment of program goals, and the definition of specific breeding objectives. The next step is selection of germplasm that possesses the traits to meet the program goals. The goal is to combine in a single variety an improved combination of desirable traits from the parental germplasm.
In lettuce, these important traits may include increased head size and weight, higher seed yield, improved color, resistance to diseases and insects, tolerance to drought and heat, better post-harvest shelf-life of the leaves, better standing ability in the field, better uniformity, and better agronomic quality.
Most cultivated forms of lettuce belong to the highly polymorphic species Lactuca sativa which is grown for its edible head and leaves. As a crop, lettuce is grown commercially wherever environmental conditions permit the production of an economically viable yield. Lettuce is the world's most popular salad. In the United States, the principal growing regions are California and Arizona which produce approximately 329,000 acres out of a total annual acreage of more than 333,000 acres (USDA, 2005). Fresh lettuce is available in the United States year-round although the greatest supply is from May through October. For planting purposes, the lettuce season is typically divided into three categories, early, mid and late, with the coastal areas planting from January to August, and the desert regions from August to December. Lettuce is consumed nearly exclusively as fresh, raw product, and occasionally as a cooked vegetable.
Lactuca sativa is in the Cichoreae tribe of the Asteraceae (Compositae family). Lettuce is related to chicory, sunflower, aster, dandelion, artichoke, and chrysanthemum. Sativa is one of about 300 species in the genus Lactuca. There are several morphological types of lettuce. The Crisphead group includes the Iceberg and Batavian types. Iceberg lettuce has a large, firm head with a crisp texture and a white or creamy yellow interior. Batavian lettuce predates Iceberg lettuce and has a smaller and less firm head. The Butterhead group has a small, soft head with an almost oily texture. Romaine lettuce, also known as Cos lettuce, has elongated upright leaves forming a loose, loaf-shaped head and the outer leaves are usually dark green. Leaf lettuce comes in many varieties, none of which form a head. There are three types of lettuce which are seldom seen in the United States: Latin lettuce, which looks like a cross between Romaine and Butterhead; Stem lettuce, which has long, narrow leaves and thick, edible stems; and Oilseed lettuce, which is a primitive type of lettuce grown for its large seeds that are pressed to obtain oil.
Lactuca sativa is a simple diploid species with nine pairs of chromosomes (2N=18). Lettuce is an obligate self-pollinating species which means that pollen is shed before stigma emergence, assuring 100% self-fertilization. Since each lettuce flower is an aggregate of about 10-20 individual florets (typical of the Compositae family), manual removal of the anther tubes containing the pollen is tedious. As a result, a modified method of misting to wash off the pollen prior to fertilization is needed to assure crossing or hybridization. Flowers to be used for crossings are selected about 60-90 minutes after sunrise. Selection criteria include plants with open flowers, where the stigma has emerged and pollen is visibly attached to a single stigma (there are about 10-20 stigma). Pollen grains are washed off using 3-4 pumps of water from a spray bottle and with enough pressure to dislodge the pollen grains without damaging the style. Excess water is then dried off using clean paper towels and about 30 minutes later, the styles spring back up and the two lobes of the stigma are visibly open in a “V” shape. Pollen from another variety or donor parent is then introduced by gently rubbing the stigma and style of the donor parent to the maternal parent. Most pertinent information including dates and pedigree are then secured to the flowers using tags.
Choice of breeding or selection methods depends on the mode of plant reproduction, the heritability of the trait(s) being improved, and the type of cultivar used commercially (e.g., F1 hybrid cultivar, pureline cultivar, etc.). For highly heritable traits, a choice of superior individual plants evaluated at a single location may be effective, whereas for traits with low heritability, selection should be based on mean values obtained from replicated evaluations of families of related plants. Popular selection methods commonly include pedigree selection, modified pedigree selection, mass selection, recurrent selection and backcross breeding.
The complexity of inheritance influences choice of the breeding method. Backcross breeding is used to transfer one or a few favorable genes for a highly heritable trait into a desirable cultivar. This approach has been used extensively for breeding disease-resistant cultivars. Nevertheless, it is also suitable for the adjustment and selection of morphological character, color characteristics and simply inherited quantitative characters. Various recurrent selection techniques are used to improve quantitatively inherited traits controlled by numerous genes. The use of recurrent selection in self-pollinating crops depends on the ease of pollination, the frequency of successful hybrids from each pollination and the number of hybrid offspring from each successful cross.
Each breeding program should include a periodic, objective evaluation of the efficiency of the breeding procedure. Evaluation criteria vary depending on the goal and objectives, but should include gain from selection per year based on comparisons to an appropriate standard, overall value of the advanced breeding lines, and number of successful cultivars produced per unit of input (e.g., per year, per dollar expended, etc.).
Promising advanced breeding lines are thoroughly tested and compared to appropriate standards in environments representative of the commercial target area(s) for three or more years. The best lines are candidates for new commercial cultivars. Those still deficient in a few traits may be used as parents to produce new populations for further selection.
These processes, which lead to the final step of marketing and distribution, usually take from eight to twelve years from the time the first cross is made. Therefore, development of new cultivars is a time-consuming process that requires precise forward planning, efficient use of resources, and a minimum of changes in direction.
A most difficult task is the identification of individuals that are genetically superior, because for most traits the true genotypic value is masked by other confounding plant traits or environmental factors. One method of identifying a superior plant is to observe its performance relative to other experimental plants and/or to a widely grown standard cultivar. If a single observation is inconclusive, replicated observations provide a better estimate of its genetic worth.
The goal of lettuce plant breeding is to develop new, unique and superior lettuce cultivars. The breeder initially selects and crosses two or more parental lines, followed by repeated selfing and selection, producing many new genetic combinations. Another method used to develop new and unique lettuce cultivar occurs when the lettuce breeder selects and crosses parental varieties followed by haploid induction and chromosome doubling that result in the development of dihaploid cultivars. The breeder can theoretically generate billions of different genetic combinations via crossing, selfing and mutations and the same is true for the utilization of the dihaploid breeding method.
Each year, the plant breeder selects the germplasm to advance to the next generation. This germplasm is grown under unique and different geographical, climatic and soil conditions and further selections are then made during and at the end of the growing season. The cultivars that are developed are unpredictable. This unpredictability is because the breeder's selection occurs in unique environments with no control at the DNA level (using conventional breeding procedures or dihaploid breeding procedures), and with millions of different possible genetic combinations being generated. A breeder of ordinary skill in the art cannot predict the final resulting lines he develops, except possibly in a very gross and general fashion. This unpredictability results in the expenditure of large amounts of research monies to develop superior new lettuce cultivars.
The development of new lettuce cultivars requires the development and selection of lettuce varieties, the crossing of these varieties and the evaluation of the crosses.
Pedigree breeding and recurrent selection breeding methods are used to develop cultivars from breeding populations. Breeding programs combine desirable traits from two or more cultivars or various broad-based sources into breeding pools from which cultivars of desired phenotypes are developed by selfing and selection or through the dihaploid breeding method.
Pedigree breeding is used commonly for the improvement of self-pollinating crops or inbred lines of cross-pollinating crops. Two parents that possess favorable, complementary traits are crossed to produce an F1. An F2 population is produced by selfing one or several F1s or by intercrossing two F1s (sib mating). The dihaploid breeding method could also be used. Selection of the best individuals may begin in the F2 population; then, beginning in the F3, the best individuals in the best families are selected. Replicated testing of families, or hybrid combinations involving individuals of these families, often follows in the F4 generation to improve the effectiveness of selection for traits with low heritability. At an advanced stage of inbreeding (i.e., F6 and F7), the best lines or mixtures of phenotypically similar lines are tested for potential release as new cultivars.
Mass and recurrent selections can be used to improve populations of either self- or cross-pollinating crops. A genetically variable population of heterozygous individuals is either identified, or created, by intercrossing several different parents. The best plants are selected based on individual superiority, outstanding progeny, or excellent combining ability. The selected plants are intercrossed to produce a new population in which further cycles of selection are continued.
Backcross breeding has been used to transfer genes for a simply inherited, highly heritable trait into a desirable homozygous cultivar or inbred line which is the recurrent parent. The source of the trait to be transferred is called the donor parent. The resulting plant is expected to have the attributes of the recurrent parent (e.g., cultivar) and the desirable trait transferred from the donor parent. After the initial cross, individuals possessing the phenotype of the donor parent are selected and repeatedly crossed (backcrossed) to the recurrent parent. The resulting plant is expected to have the attributes of the recurrent parent (e.g., cultivar) and the desirable trait transferred from the donor parent.
The single-seed descent procedure in the strict sense refers to planting a segregating population, harvesting a sample of one seed per plant, and using the one-seed sample to plant the next generation. When the population has been advanced from the F2 to the desired level of inbreeding, the plants from which lines are derived will each trace to different F2 individuals. The number of plants in a population declines each generation due to failure of some seeds to germinate or some plants to produce at least one seed. As a result, not all of the F2 plants originally sampled in the population will be represented by a progeny when generation advance is completed.
In a multiple-seed procedure, lettuce breeders commonly harvest one or more flower containing seed from each plant in a population and thresh them together to form a bulk. Part of the bulk is used to plant the next generation and part is put in reserve. The procedure has been referred to as modified single-seed descent or the bulk technique.
The multiple-seed procedure has been used to save labor at harvest. It is considerably faster than to remove one seed from each by hand for the single-seed procedure. The multiple-seed procedure also makes it possible to plant the same number of seeds of a population each generation of inbreeding. Enough seeds are harvested to make up for those plants that did not germinate or produce seed.
Descriptions of other breeding methods that are commonly used for different traits and crops can be found in one of several reference books (e.g., R. W. Allard, 1960, Principles of Plant Breeding, John Wiley and Son, pp. 115-161; N. W. Simmonds, 1979, Principles of Crop Improvement, Longman Group Limited; W. R. Fehr, 1987, Principles of Crop Development, Macmillan Publishing Co.; N. F. Jensen, 1988, Plant Breeding Methodology, John Wiley & Sons).
Proper testing should detect any major faults and establish the level of superiority or improvement over current cultivars. In addition to showing superior performance, there must be a demand for a new cultivar that is compatible with industry standards or which creates a new market. The introduction of a new cultivar will incur additional costs to the seed producer, the grower, processor and consumer for special advertising and marketing, altered seed and commercial production practices, and new product utilization. The testing preceding release of a new cultivar should take into consideration research and development costs as well as technical superiority of the final cultivar. For seed-propagated cultivars, it must be feasible to produce seed easily and economically.
Lettuce in general and iceberg lettuce in particular, is an important and valuable vegetable crop. Thus, a continuing goal of plant breeders is to develop stable, high yielding lettuce cultivars that are agronomically sound. The reasons for this goal are obviously to maximize the amount of yield produced on the land. To accomplish this goal, the lettuce breeder must select and develop lettuce plants that have the traits that result in superior cultivars.
The foregoing examples of the related art and limitations related therewith are intended to be illustrative and not exclusive. Other limitations of the related art will become apparent to those of skill in the art upon a reading of the specification.